Gas valves for ion guides

ABSTRACT

An ion optical arrangement (1) for use in a mass spectrometer comprises electrodes (11) defining an ion optical path, a housing (18) for accommodating the electrodes, a voltage source for providing voltages to the electrodes to produce electric fields, and a valve for allowing gas to enter and/or leave the housing. The valve comprises an electrostatic mechanism and/or a pneumatic mechanism. The electrostatic mechanism may comprise a flexible foil (30, 31) configured for covering at least one opening (16) in the ion optical arrangement when a first voltage is applied and being spaced apart from the at least one opening when a second voltage is applied. The pneumatic mechanism may comprise a Bourdon tube.

FIELD OF THE INVENTION

The present invention relates to isotope ratio mass spectrometry (MS).More in particular, the present invention may relate to interferencefree, high resolution, multi-collector isotope ratio mass spectrometryand elemental analysis, for example in combination with a collision celland a plasma source, such as an inductively coupled plasma (ICP) source.

BACKGROUND OF THE INVENTION

Multi-collector ICP-MS is an established method for high precision andaccurate isotope ratio analysis. Applications are in the field ofgeochronology, geochemistry, cosmochemistry, biogeochemistry,environmental sciences as well as in life sciences. Precise and accurateisotope ratio measurements very often provide the only information togain deeper insight into scientific questions which cannot be answeredby any other analytical technique. However, elemental and molecularinterferences in the mass spectrometer limit the attainable precisionand accuracy of the analysis.

These interferences are present in the sample material itself or aregenerated by sample preparation from a contamination source (usedchemicals, cleanliness of sample container, and fractionation duringsample purification) or are even generated in the ion source or in themass spectrometer. The problems with such interferences can be counteredby:

-   1. using a high mass resolution mass analyzer that discriminates    against interferences by detecting small differences in mass of the    interference relative to the sample ion;-   2. by sample preparation and chemical separation of interference    prior to mass analysis; and/or-   3. by using a collision cell integrated into the mass analyzer.    In a collision cell the chemical interferences are removed by    chemical reactions, or by kinetic energy discrimination, taking    advantage of different cross sections of molecular and elemental    species inside the pressurized collision cell which results into    different kinetic energy losses of molecular and elemental ions. By    means of a high pass energy filter following the collision cell the    lower energy molecular species can be discriminated.

A collision cell is an encapsulated volume within the ion optical beampath which is pressurized with a collision gas to cause interactions(i.e. collisions and/or chemical reactions between the ions and the gasmolecules). In order to generate efficient collisions and chemicalreactions inside the collision cell, the ions preferably are at a lowion beam energy of a few electronvolt (eV) only. The collision cellusually is a multipole ion guide which is powered by RF fields to guidethe ions through the collision cell. In order to achieve a reasonablegas pressure, the multipole ion guide is encapsulated in a compactvolume with small entrance and entrance apertures, typically in therange of 1-3 mm diameter. A collision cell coupled to a multi-collectormass spectrometer is disclosed in British patent application GB 2 546060 (Thermo Fisher Scientific (Bremen) & The University of Bristol).

Ions having different masses but the same energy travel at differentvelocities through the time dependent oscillating field of the collisioncell and as a result the ion trajectories are mass dependent. In otherwords, the trajectories depend on the mass of the ions traveling throughthe RF field. This effect is called “noding”. This can in particularpose a problem at the exit of the multipole structure, where ions ofdifferent masses may exit at different angles.

The mass dependence of the collision cell transmission can be a problemfor accurate isotope ratio measurements, even when it is small. However,for some analytical applications there is no other choice to removeisobar interferences but to use the collision cell.

For samples where no interferences are present it would be advantageousto avoid the low energy passage of the ions through the radio frequency(RF) multipole collision cell optics and to exclude any uncertainty ofthe discrimination effects caused in the collision cell (i.e. chemicaleffects as well as the noding effect).

It is noted that the undesired “noding effect” is not limited tocollision cells but may also occur in other ion optical arrangements,such as mass filters.

One way to solve this problem is to install a second beam path in themass spectrometer where the ion beam is deflected off axis prior to thecollision cell to bypass the collision cell and finally to deflect theions back onto the optical axis of the mass spectrometer. Such a dualpath ion optics arrangement is described in British patent applicationGB 2 535 754 (Nu Instruments). It allows to switch between the lowenergy collision cell beam path and an off axis static high energy beampath. This results into a rather complicated setup with several ion beamdeflectors causing image aberrations and alignment problems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ion opticalarrangement, such as a collision cell or a mass filter, for a massspectrometer which can largely avoid the noding problems related toexisting RF driven ion optics and which is simpler and more compact thanthe dual path arrangement of the prior art. This object may be achievedby a collision cell which has two operating modes: a pressurized mode inwhich a collision gas is present, and a vacuum mode in whichsubstantially no collision gas is present. However, this requires arapid switching between the two modes, and therefore a rapidpressurizing and emptying of the collision cell. This in turn requires avalve that can operate very quickly and efficiently.

It is therefore a further object of the present invention to provide avalve for use in an ion optical arrangement, such as but not limited toa collision cell, which valve can be switched very rapidly.

Accordingly, the present invention provides an ion optical arrangementfor use in a mass spectrometer, comprising:

-   -   electrodes defining an ion optical path,    -   a housing for accommodating the electrodes,    -   a voltage source for providing voltages to the electrodes to        produce electric fields, and    -   a valve for allowing gas to enter and/or leave the housing,        wherein the valve comprises an electrostatic mechanism.

By providing an ion optical arrangement which has an electrostatic valvemechanism, the housing of the ion optical arrangement can be quicklyopened or closed, thus avoiding conventional valves. As in someapplications the primary function of a valve is to quickly releasepressure, a valve in an ion optical arrangement may also be referred toas pressure release mechanism.

The electrostatic mechanism may comprise a flexible foil configured forcovering at least one opening in the ion optical arrangement when afirst voltage is applied and for being spaced apart from the at leastone opening when a second voltage is applied. A flexible foil, which mayhave a thickness of less than 1 mm, preferably less than 0.2 mm, hasvery little mass and can quickly be moved.

The flexible foil may comprise at least one insulating layer and atleast one conducting layer. In some embodiments, the flexible foil maycomprise only a conducting layer. Typically, two insulating layers maybe used to insulate the conducting layer from the conducting outer wallof the housing and from the conducting support element. Each of thesetwo insulating layers may be attached to either the conducting layer orone of the housing and the support element. Flexible foils having morethan one conductive layer, for example two or three conductive layersseparated by insulating layers may also be used.

In an embodiment, the flexible foil is arranged in a spacing between thehousing and a support element. That is, the flexible foil may cover awall part of the housing which contains one or more openings. Thesupport element may be a plate which preferably has a similar shape tothe wall part of the housing so as to provide a substantially uniformspacing. The support element may therefore be flat or curved dependenton the shape of the housing. The support element is at least partiallyelectrically conducting.

The ion optical arrangement according to the invention may furthercomprise a pump for pressurizing the ion optical arrangement. The pumpmay for example be used to pressurize the ion optical arrangement atleast during a first operation mode in which it is used as a collisioncell. The pressurizing pump may be switched off in a second operationmode in which no collision gas is used. In some embodiments, the pumpmay be reversed in the second operation mode.

In an embodiment, the ion optical arrangement may comprise a switchablepumping cross section in the collision cell housing for establishing ahigher gas pressure inside the first operation mode (low cross section)and pumping the collision cell efficiently in the second operation mode(high cross section). The first operation mode may be a low energy modewhile the second operation mode may be a high energy mode. That is, theions passing through the collision cell may have a relatively low energyin the first operation mode when gas is present and a relatively highenergy in the second operation mode, when virtually no gas is present.

The invention also provides an electrostatic valve for use in an ionoptical arrangement, wherein the electrostatic valve comprises aflexible foil configured for covering at least one opening in the ionoptical arrangement when a first voltage is applied and being spacedapart from the at least one opening when a second voltage is applied.

The invention further provides an ion optical arrangement for use in amass spectrometer comprising:

-   -   electrodes defining an ion optical path,    -   a housing for accommodating the electrodes,    -   a voltage source for providing voltages to the electrodes to        produce electric fields, and    -   a valve for allowing gas to enter and/or leave the housing,        wherein the valve comprises a pneumatic mechanism.

A suitable pneumatic mechanism may provide a good alternative to anelectrostatic mechanism. In an embodiment, the pneumatic mechanismcomprises a Bourbon tube. A Bourdon tube, which is known per se,typically consists of a rounded or wound tube which straightens wheninflated. An example of a Bourdon tube is disclosed in U.S. Pat. No.3,188,419. A Bourdon tube, which can be operated by gas pressure, iscapable of switching quickly.

In an embodiment, the Bourdon tube is arranged for opening the housingwhen inflated and closing the housing when deflated. The housing of theion optical arrangement may comprise a hinged flap which is capable ofclosing off an opening in the housing, and the hinged flap beingoperated by the Bourdon tube.

The ion optical arrangement of the invention may be a collision cell ora collision/reaction cell. However, the invention is not so limited andthe ion optical arrangement of the invention may be any ion guide ofwhich the gas pressure is to be variable.

The invention additionally provides a pneumatic valve for use in an ionoptical arrangement, comprising a hinged flap arranged for openingand/or closing at least one opening in the ion optical arrangement, anda Bourbon tube arranged to operate the flap.

In another embodiment, the valve mechanism may comprise a relay so as toelectrically operate the mechanism. In some embodiments, a Bourdon tubeand a relay may advantageously be combined.

The invention further provides a mass spectrometer comprising an ionoptical arrangement as described above. The mass spectrometer accordingto the invention may further comprise at least one ion source, such asan inductively coupled plasma ion source, and at least one detectorarrangement, such as a multi-collector detector arrangement, andpreferably also a mass filter. The invention is, however, not limited tomass spectrometers having plasma sources.

It is noted that the ion optical axis along which ions pass through theion optical arrangement may be straight but that this is not essential.In some embodiments, the ion optical axis through the collision cell isstraight but the path of the ions through the ion optical arrangementmay not be straight and may be partially or entirely curved, as in thearrangement of GB 2 546 060, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a multipole collision/reaction cell in whichthe invention may be utilized.

FIGS. 2A-2C schematically show an embodiment of an electrostaticpressure release mechanism which may be used with the collision/reactioncell of FIG. 1 or with another ion optical arrangement.

FIG. 3 schematically shows an ion optical arrangement with anelectrostatic valve mechanism according to the invention.

FIGS. 4A & 4B schematically show an embodiment of a pneumatic pressurerelease mechanism which may be used with the collision/reaction cell ofFIG. 1 or with another ion optical arrangement.

FIG. 5 schematically shows a mass spectrometer comprising an ion opticalarrangement in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned above, it is an object of the present invention to allow anion optical arrangement, such as a collision/reaction cell or a massfilter, to quickly switch between a pressurized state and adepressurized or vacuum state, or between a high pressure state and alow pressure state. Such an ion optical arrangement may also be referredto as switchable ion guide.

The pressurized state may for example be a state in which a collisiongas is used, which may be a state in which the ions have a relativelylow energy. The depressurized state may be a state in which a collisiongas is not desired, which may be a state in which the ions have arelatively high energy.

When operating an ion optical arrangement, such as a collision cell, ina pressurized mode and in an evacuated (that is, non-pressurized) mode,it is typically required that the ion optical arrangement can bepressurized and depressurized rapidly. In particular, a pressure releasemechanism is desired that is fast and effective.

FIG. 1 schematically shows an ion optical arrangement in which theinvention may be applied. The collision cell 1 is shown to comprise ahousing 18 in which a multipole arrangement is accommodated. In theexample shown, the multipole arrangement is a hexapole arrangementcomprising six elongate poles or rods 11 which constitute electrodes.The multipole arrangement has an axis of symmetry which is the ionoptical axis. A radio frequency (RF) voltage may be fed to oppositepairs of poles 11 to produce an RF electric field. Ions can enter thecollision cell through an entrance aperture 13 and leave the collisioncell through an exit aperture 15. The RF field produced by the multipolearrangement focuses the ions on the longitudinal axis of thearrangement. This is particularly relevant when a collision gas ispresent in the collision cell, as collisions may cause the ions todeviate from their path.

The invention provides valve mechanisms which are particularly suitablefor use in a collision cell or other ion guide having a pressurized andan evacuated operation mode.

FIG. 2A schematically shows an electrostatic valve mechanism which maybe used in a collision cell, for example. The exemplary collision cell 1is shown to comprise a housing 18 in which rods 11 are accommodated. Anion beam IB can pass through the collision cell 1, through openings inthe front plate 12 and back plate 14 respectively. In the embodimentshown, part of the wall of the housing 18 is provided with through holes16 which can be closed off by a movable foil. This foil is located in aspacing between the housing 18 and a support element 19, which is hereconstituted by a plate. Both the housing 18 and the plate 19 containelectrically conductive material and may both be made of metal, or atleast contain a metal layer or other conductive layer. The plate 19,which extends substantially parallel to the housing 18, may be flat butmay alternatively be curved to accommodate any curvature of the housing18.

In the embodiment shown, the foil comprises two layers: a conductivelayer 30 and an electrically insulating layer 31. A further electricallyinsulating layer 32 is attached to the plate 19. In an alternativeembodiment, the foil consists of a single layer: the conductive layer 30only, in which case the insulating layers 31 and 32 are permanentlyattached to the housing 18 and the plate 19 respectively. In yet analternative embodiment, the foil consists of three layers: theconductive layer 30 and both insulating layers 31 & 32. Further layersmay be added, as long as the foil remains sufficiently flexible. Asuitable material for the insulating layers 31 & 32 is Kapton, but othermaterials, for example other polyimides, may also be used. Theconductive layer may be made of copper foil, for example.

As mentioned above, the flexible foil is located in the spacing betweenthe housing 18 and the plate 19. One edge of the foil may be attached tothe housing 18 while the opposite edge may be attached to the plate 19,such that the foil bridges the spacing. By applying DC voltages to theconductive layer, the position of the foils can be changed, as shown inFIG. 9A by the arrows which indicate the possible movement of thesubstantially S-shaped spacing-bridging portion of the foil.

Referring to FIG. 2B, the housing 18 will typically be connected toground (GND). The conductive plate 19 can be connected to a highvoltage, indicated by HV in FIG. 9B, thus creating a voltage differenceover the spacing between the housing 18 and the plate 19. If theconductive layer 30 is connected to a high voltage, then the foil willbe repelled by the plate 19 and attracted by the housing 18. As aconsequence, the foil will tend to move towards the housing and theS-shaped spacing bridging part will move to the right (see also FIG.2A). In other words, electrical forces F_(el) pulling the foil towardsthe housing cause a mechanical force F_(m) to the right in FIG. 2B. Thefoil will cover the through holes 16 and the interior of the collisioncell will be closed off.

Referring to FIG. 2C, the through holes 16 can be opened by connectingthe conductive layer 30 to ground instead of to the high voltage (HV).This will cause the foil to be repelled by the housing 18 and to beattracted by the plate 19, which in turn cause the S-shaped spacingbridging part to move to the left (see also FIG. 2A). In other words,electrical forces F_(el) pulling the foil towards the plate 19 cause amechanical force F_(m) to the left in FIG. 2C. The foil will no longercover the through holes 16 and the interior of the collision cell willbe open to the surrounding atmosphere.

As the movement of the foil is controlled by voltages, which can beswitched extremely quickly, and as the foil can have a very low mass,the movement of the foil can be very quick. Accordingly, the pressureinside the collision cell 1 can be adjusted very rapidly and switchingbetween a pressurized state and an evacuated state can be carried outalmost instantly.

An embodiment of the electrostatic valve is shown in perspective in FIG.3, together with an ion guide and electric circuitry for operating theelectrostatic valve. The ion guide is shown to have a housing 18 with anentrance opening 13 for ions. A flexible foil arrangement is shown tocomprise a conductive layer 30, a first insulating layer 31 and a secondinsulating layer 32. In the embodiment shown, the insulation layers 31and 32 are permanently attached to the housing 18 and the plate 19respectively, the conductive layer 30 being the only movable layer. Insome embodiments, two or more conductive layers may be used, separatedby additional insulating layers.

In the embodiment shown, the openings 16 in the housing are also presentin the first insulating layer 31. In embodiments where the firstinsulating layer 31 is not attached to the housing but to the conductivelayer 30, the openings 16 may not be present in the first insulatinglayer 31.

FIG. 3 also schematically shows an electric circuit for operating theelectrostatic valve. A voltage source 40 produces a voltage U. A switch42 allows the conductive layer 30 to connect either to ground (as shown)or to the voltage U. The plate 19 is shown to be permanently connectedto the voltage U. If the conductive layer 30 is connected to ground, asshown, it will be attracted to the plate 19 and move towards the plate.Conversely, if the conductive layer 30 is connected to the voltagesource 40, then the conductive layer 30 will be repulsed by the plate 19and move towards the housing 18, thus closing the openings 16.

FIGS. 4A & 4B show a pneumatic mechanism 20 for adjusting the pumpingcross section of an ion guide, for example a collision cell housing 18having rods 11. The pneumatic valve mechanism 20 is shown to comprise adoor or flap 21 which is connected via a hinge 22 to the housing 18 ofthe collision cell 1. The flap 21 can be operated by an actuator 23 ofwhich one end is connected to the flap 21 and the other end is connectedto a support element 24 attached to the housing 18.

The actuator 23 shown in FIGS. 4A & 4B is a Bourdon tube. A Bourdon tubecomprises a bent tube. The bending radius of the bent tube can bedecreased if the pressure difference between the inner part and theouter part of tube increases. To this end, a gas tube 25, which is alsoconnected to the support element 24, is connected with the actuator 23.In the embodiment shown, the gas flows from the gas tube 25 through achannel in the support element 24 into the actuator 23 when the gaspressure in the gas tube 25 is higher than the gas pressure surroundingthe actuator 23. By letting gas flow into the actuator, its bendingradius decreases (the actuator straightens) and the flap is opened.Conversely, the gas flows from the actuator 23 through the supportelement 24 into the gas tube 25 when the gas pressure in the gas tube 25is lower than in the actuator 23. By letting gas flow out of theactuator, its bending radius increases (the actuator curves) and theflap is closed.

Thus, by providing a pressure difference between the gas tube 25 and theair (or other gas) outside the actuator 23, the flap can be quicklyopened or closed, thus allowing the gas pressure in the interior of thecollision cell 1 to quickly assume the gas pressure on its outside.

It is noted that the collision cell 1 may be accommodated in anear-vacuum environment, while the gas tube may be connected with anenvironment under atmospheric pressure. The gas used for inflating theinflatable actuator may be air. As the interior volume of the actuator23 and the gas tube 25 may be small, only a small amount of air or othergas is needed to inflate the actuator. This air or other gas may beprovided by a gas reservoir or by a pump. Thus, a small pump or valvecan be sufficient to indirectly operate the relatively large flap.

By using a Bourdon tube or similar actuator, a fast and effectivepressure regulation of a collision cell or other ion guide can beachieved.

The exemplary mass spectrometer 10 schematically shown in FIG. 5comprises a multipole cell 1, which can be a collision cell as describedabove but which can be replaced by an ion guide without a multipolearrangement. The mass spectrometer 10 may further comprise a plasmasource 1, such as an ICP (inductively coupled plasma) source forgenerating an ion beam IB1. The mass spectrometer may further comprise amass filter 3, such as a magnetic sector mass filter. In the magneticsector mass filter, the ion beam 1131 is separated into partial beamsIB2 having different m/z (mass versus charge) ratios, which partialbeams can be detected by the detector assembly 4, which may be amultiple detector assembly. The mass spectrometer 10 may furthercomprise a pump for lowering the gas pressure in the collision cell 1, avalve associated with the pump, a voltage source 5 for supplying DC andAC (RF) voltages to the collision cell 1, and a controller forcontrolling the various components of the mass spectrometer 10. Thevoltage source 5 may correspond with the voltage source 40 in FIG. 3.The valve may comprise a foil-based valve and/or a Bourdon tube-basedvalve as described above.

Aspects of the invention comprise:

-   a) An ion guide, such as a multipole collision cell, which can be    rapidly switched between a first operation mode, in which a    collision gas and/or reaction gas is present, and a second operation    mode, in which no gas is used.-   b) Mechanisms for allowing a rapid switch between the first    operation mode and the second operation mode.    These aspects of the invention may be used in isolation or in    combination.

Although the invention has been described above mainly with reference toan ion optical arrangement comprising a multipole, such as a hexapole,the invention is not so limited and may also be utilized in other typesof ion guides.

It will therefore be understood by those skilled in the art that theinvention is not limited to the embodiments shown and that manyadditions and/or modifications can be made without departing from thescope of the invention as defined in the appending claims.

1. An ion optical arrangement for use in a mass spectrometer,comprising: electrodes defining an ion optical path, a housing foraccommodating the electrodes, a voltage source for providing voltages tothe electrodes to produce electric fields, and a valve for allowing gasto enter and/or leave the housing, wherein the valve comprises anelectrostatic mechanism.
 2. The ion optical arrangement according toclaim 1, wherein the electrostatic mechanism comprises a flexible foilconfigured for covering at least one opening in the ion opticalarrangement when a first voltage is applied and being spaced apart fromthe at least one opening when a second voltage is applied.
 3. The ionoptical arrangement according to claim 2, wherein the flexible foilcomprises at least one insulating layer and at least one conductinglayer.
 4. The ion optical arrangement according to claim 2, wherein theflexible foil is arranged in a spacing between the housing and a supportelement.
 5. The ion optical arrangement according to claim 1, furthercomprising a pump for pressurizing the ion optical arrangement.
 6. Anelectrostatic valve for use in an ion optical arrangement, wherein theelectrostatic valve comprises a flexible foil configured for covering atleast one opening in the ion optical arrangement when a first voltage isapplied and being spaced apart from the at least one opening when asecond voltage is applied.
 7. An ion optical arrangement for use in amass spectrometer comprising: electrodes defining an ion optical path, ahousing for accommodating the electrodes, a voltage source for providingvoltages to the electrodes to produce electric fields, and a valve forallowing gas to enter and/or leave the housing, wherein the valvecomprises a pneumatic mechanism.
 8. The ion optical arrangementaccording to claim 7, wherein the pneumatic mechanism comprises aBourbon tube.
 9. The ion optical arrangement according to claim 8,wherein the Bourdon tube is arranged for opening the housing wheninflated and closing the housing when deflated.
 10. The ion opticalarrangement according to claim 9, wherein the housing comprises a hingedflap which is capable of closing off an opening in the housing, andwherein the hinged flap is operated by the Bourdon tube.
 11. The ionoptical arrangement according to claim 7, further comprising a pump forpressurizing the ion optical arrangement.
 12. The ion opticalarrangement according to claim 1, which is a collision cell or acollision/reaction cell.
 13. A pneumatic valve for use in an ion opticalarrangement, comprising a hinged flap arranged for opening and/orclosing at least one opening in the ion optical arrangement, and aBourbon tube arranged to operate the flap.
 14. A mass spectrometercomprising an ion optical arrangement according to claim
 1. 15. The massspectrometer according to claim 13, further comprising at least one ionsource, such as an inductively coupled plasma ion source, and at leastone detector arrangement, such as a multi-collector detectorarrangement, and preferably also a mass filter.
 16. The ion opticalarrangement according to claim 7, which is a collision cell or acollision/reaction cell.
 17. A mass spectrometer comprising an ionoptical arrangement according to claim 7.